I had a slow leak on my kite so I did the soapy water trick and found a hole. Fixed it.
Leading edge back in.
Pump it again.
Slow leak again but this time very slow: instead of 1 hour, I'd say that it stays decently firm for 3 hours or so.
Struts were clipped so the problem doesn't seem there.
I read here that it could be a valve problem.

Had a similar issue a while ago. Drove me crazy. Dragged the whole kite into my pool and found following:

Check inflate/deflate valve plugs. After spraying them with dish soap I could see fine bubbling AFTER 5 min between valve throat and plug. Gave the plug one (1) wrap with Teflon tape, glued the tape and viola ...

Also one of my air tubes connecting LE and strut was slowly, very slowly leaking. These tubes are fastened with zip ties to outlets on either end. Obviously some of these zip ties get loose over time and allow a small air leak.

Had same problem, also with a Cab kite, but my problem was not brand related. Some sand had gotten between the bladder and leading edge. On a microscopic scale, the particular type of sand was sharp, and would make new super-small very difficult to find hole(s) each time I re-assembled and inflated the bladder. I repaired it like four times until I decided to violently shake any sand out (with bladder removed) and repair the last hole. It's been good since.

The clue to me was that these pin-holes would keep showing up on the same side of the bladder relative to the inflation point.

This may or may not be your problem, but sand or any other debris between the bladder and leading edge is bad news no matter what.

Check to be sure the sealing washer (rubber washer on bottom of valve that goes over the male threaded end) didn't blow off. It's easy to miss because it blends in well with the valve. This has happened to me twice now when deflating Cabs - the initial blast of air causes it to fly. I never realized it was gone until next session when kite was getting soft after an hour or so. Now I hold my hand over the valve while unscrewing to catch it if it does happen again.

The Airlock valve was upgraded in recent years (but I cant remember what year) where they modified the non-return flap so that there was less resistance when pumping up the kite, so if you have one of the older valves, its worth upgrading for the easier pumping.

They have now upgraded the Airlock valve again for 2013 so it has a cap and doesn't need an adaptor.

If you want to try the inexpensive solution first then go buy some plumber's tape (aka teflon tape) at your local hardware store and wrap it around your valve plug. It creates an air tight seal. If that stops the leak then you know it is your valve. Plumber's tape cost about 30 cents a roll.

I haven't seen this on many Cabrinhas but you never know when you get a bad bladder. I worked on a Crazy fly for 3 days taking the bladder out patching holes and so on. I found that the bladder was old, and I could see where the rubber was starting to crack. They weren't holes leaking air even when pumped up and squeezing the section of bladder to try to force air through. However, if I stretched the area a bit, pulling the rubber perpendicular to the line of the crack, and squeezed the tube I would begin to see bubbles from the soapy water. Sometimes it helps to have some bright lights and holding clipboard behind the bladder. I went over this bladder carefully with out air in it, getting different view points to examine for more cracks. It took a while but I sharpie each suspected area and then got the spray bottle to verify if the crack was all the way through the bladder. Something else to look for is along the seam of the bladder. It's easy for pinholes and cracking to form and hide from scrutiny. If your bladder is still in the kite, pump it up and dunk it in the water, if you see bubbles, narrow down between which struts, or using soapy water, spray it and ifind the leak, dry it with a towel and use a fresh sharpie to try to get the ink to go through the nylon to the bladder so its easier to get close to the leak.

I haven't seen this on many Cabrinhas but you never know when you get a bad bladder. I worked on a Crazy fly for 3 days taking the bladder out patching holes and so on. I found that the bladder was old, and I could see where the rubber was starting to crack. They weren't holes leaking air even when pumped up and squeezing the section of bladder to try to force air through. However, if I stretched the area a bit, pulling the rubber perpendicular to the line of the crack, and squeezed the tube I would begin to see bubbles from the soapy water. Sometimes it helps to have some bright lights and holding clipboard behind the bladder. I went over this bladder carefully with out air in it, getting different view points to examine for more cracks. It took a while but I sharpie each suspected area and then got the spray bottle to verify if the crack was all the way through the bladder. Something else to look for is along the seam of the bladder. It's easy for pinholes and cracking to form and hide from scrutiny. If your bladder is still in the kite, pump it up and dunk it in the water, if you see bubbles, narrow down between which struts, or using soapy water, spray it and ifind the leak, dry it with a towel and use a fresh sharpie to try to get the ink to go through the nylon to the bladder so its easier to get close to the leak.